Campbell and others have hypothesized that certain visual cells in the brain respond to periodic gratings of only the specific spatial frequency to which they are "tuned"; i.e., that the brain performs a Fourier analysis of the visual world. This bold idea has won many adherents and has inspired a growing body of psychophysical literature. But (unlike many other types of feature-detecting cells) these alleged harmonic-analyzer units have not been detected electrophysiologically-- at least not in a form that would account for data which purport to show the fatiguing of such cells over very long narrow frequency bands. On the other hand, it has been shown that periodic gratings play a unique role in the normal, rapid adaptation of the retina, leaving an extraordinarily persistent after-inage whose formation is relatively insensitive to voluntary eye-movements. This raises the disconcerting possibility that the narrow-band phychophysical data are really artifacts, resulting from incomplete control of the luminance adaptation of the retina. The research proposed herein seeks to explore this possibility. We will distinguish between retinal and cortical adaptation by means of a stabilized adapting grating. In this way, retinal adaptation can be eliminated by motion or flicker of the distal stimulus, under the experimenter's control. The test grating will be stabilized in some studies. We believe the proposed series of experiments to be crucial in this area.